This invention relates to jet engines. More particularly, this invention relates to a novel method and apparatus for varying the effective fluid flow area defined at the exhaust nozzle throat of a jet engine.
Jet engines provide motive power by accelerating a flow of high-temperature, high-pressure motive fluid through an exhaust nozzle to create a high-velocity fluid jet. The thrust provided by the fluid jet is approximately equal to its mass flow rate multiplied by its velocity. During high-power operation of the jet engine the throat of the exhaust nozzle must define an area sufficiently large to accommodate the mass flow of the fluid jet without undue restriction. However, during intermediate-power operation of the engine, during which the mass flow rate of motive fluid is reduced, the effective area of the exhaust nozzle throat must be reduced in order to accelerate the reduced flow of motive fluid to an adequate velocity. Thus, in order for a jet engine to efficiently provide varying thrust levels, the effective area of the exhaust nozzle throat of the engine must be variable.
A conventional method of varying the area of a jet engine exhaust nozzle throat is to provide the core structure of the engine with an axially movable tail piece. The tail piece is telescopically arranged with the remainder of the engine core structure and is movable in and out of the exhaust nozzle throat to vary the effective area of the nozzle throat. U.S. Pat. No. 2,405,723, granted 13 August 1946 to S. Way illustrates a jet engine having a movable tail piece.
A jet engine having a movable tail piece has a number of recognized deficiencies. Among these recognized deficiencies is the necessity for the core structure of the engine to extend to and through the exhaust nozzle throat. As a result, when the engine includes a long tail pipe the core structure of the engine must be made undesirably long. Therefore, the weight of the engine may be undesirably increased.